Four-directional forklift truck

ABSTRACT

A four-directional forklift truck comprising a chassis  10  having two front wheels  12 L,  12 R and one rear wheel  14.  A respective hydrostatic motor  18  drives each wheel selectively in either one of two opposite directions of rotation. The truck includes control means for placing the truck in a carousel mode wherein the three wheels are set at respective steering angles in which their axes of rotation A 1,  A 2,  A 3  intersect at substantially common vertical axis C equidistant from each wheel and all three wheels are driven. A switchover valve  30,  which is connected between one of the front wheels  12 R and a hydraulic power supply  22  and is actuated in sideways mode, is de-actuated in carousel mode.

FIELD OF THE INVENTION

This invention relates to a four-directional forklift truck, i.e. aforklift truck capable of being driven in forward and reverse directions(standard mode) and also in left and right sideways directions (sidewaysmode). In particular, it relates to a three-wheel forklift truck withall-wheels driven.

BACKGROUND TO THE INVENTION

In addition to operation in the standard and sideways modes referred toabove, it is also desirable to be able to operate such a truck incarousel mode, i.e. where it rotates substantially on the spot.

However, due to limitations in existing three-wheel drive machines,especially those using hydrostatic motors to drive the wheels, up to thepresent time manufacturers have had to accept a large turning radiuscentred around the single rear drive wheel. Alternatively they havetried to convert the machine into a two-wheel drive when operating incarousel mode by disconnecting the drive to one of the front wheels.Alternatively they have tried to convert the machine to one-wheel driveby disconnecting the drive to the two front wheels.

In either case this has required the extra cost of providingdisconnection valves in one or more of the hydraulic circuits to thewheel motor(s). Also, when hydraulic fluid is disengaged from one ormore wheel motor(s) in a hydrostatic circuit, cavitation can occur inthe non-driven motor(s) when the other wheel(s) are being driven.

SUMMARY OF THE INVENTION

The invention provides a four-directional forklift truck comprising achassis having two front wheels and one rear wheel, each wheel beingdirectionally adjustable by rotation about a substantially verticalaxis, and a respective motor for driving each wheel selectively ineither one of two opposite directions of rotation, wherein the truck isoperable in a carousel mode wherein the three wheels are set atrespective directions in which their axes of rotation intersect at asubstantially common vertical axis equidistant from each wheel and allthree wheels are driven, whereby the truck rotates substantially aboutthe said common vertical axis.

In a preferred embodiment the invention provides a four-directionalforklift truck comprising:

-   -   a chassis having two front wheels and one rear wheel, each wheel        being directionally adjustable by rotation about a substantially        vertical axis,    -   a respective hydrostatic motor for driving each wheel        selectively in either one of two opposite directions of        rotation, each motor having first and second hydraulic fluid        inlet ports, the application of hydraulic fluid under pressure        to the first inlet port driving the wheel in one direction and        the application of hydraulic fluid under pressure to the second        inlet port driving the wheel in the opposite direction, and    -   a hydraulic circuit comprising a source of hydraulic fluid under        pressure having first and second fluid supply ports, the        hydraulic fluid under pressure being selectively supplied at the        first or second supply port, the first and second inlet ports of        at least one front wheel being coupled to the first and second        supply ports via a respective selectively actuable switchover        valve and the first and second inlet ports of the other wheel(s)        being non-switchably coupled to the first and second supply        ports respectively,    -   wherein when the truck is operated in standard mode the        switchover valve is not actuated, the front wheels are set        substantially in the front-to-rear direction of the chassis, and        the rear wheel is steerable to turn the truck in the required        direction, and    -   wherein when the truck is operated in sideways mode the        switchover valve is actuated, the rear wheel is set        substantially normal to the front-to-rear direction of the        chassis, and the front wheels are steerable simultaneously in        opposite directions of rotation,    -   the truck further being operable in a carousel mode in which the        switchover valve is not actuated, the three wheels are set at        respective directions in which their axes of rotation intersect        at a substantially common vertical axis equidistant from each        wheel, and all three wheels are driven, whereby the truck        rotates substantially about the said common vertical axis.

BRIEF DESCRIPTION ON THE DRAWINGS

Embodiments of the invention will now be described, by way of example,with reference to the accompanying drawings, in which:

FIG. 1 is a schematic plan view of a forklift truck according to a firstembodiment of the invention, operating in standard mode;

FIG. 2 is a schematic plan view of the truck of FIG. 1 operating insideways mode;

FIG. 3 is a schematic plan view of the truck of FIG. 1 rotatinganti-clockwise in carousel mode;

FIG. 4 is a schematic plan view of the truck of FIG. 1 rotatingclockwise in carousel mode;

FIG. 5 is a schematic plan view of a forklift truck according to asecond embodiment of the invention, operating in standard mode;

FIG. 6 is a schematic plan view of the truck of FIG. 5 operating insideways mode;

FIG. 7 is a schematic plan view of the truck of FIG. 5 rotatinganti-clockwise in carousel mode; and

FIG. 8 is a schematic plan view of the truck of FIG. 5 rotatingclockwise in carousel mode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a three-wheel, four-directional forklift truck withall-wheel drive comprises a chassis 10 having left and right frontground wheels 12L, 12R respectively and a single rear ground wheel 14disposed centrally between, but rearwardly displaced, relative to thefront wheels. All three wheels are directionally adjustable by rotationabout respective substantially vertical axes 16. Such rotation iseffected by respective hydraulic cylinders associated with each wheel.This is well-known and is therefore not shown. The chassis carries aconventional mast (not shown) and lift forks 20.

Each wheel 12L, 12R and 14 can be driven selectively in either one oftwo opposite directions of rotation by a respective hydrostatic motor18. Each motor has hydraulic fluid inlet ports F and R respectively, theapplication of hydraulic fluid under pressure to the inlet port Fdriving the wheel in a forward direction (indicated by the arrows inFIG. 1) and the application of hydraulic fluid under pressure to theinlet port R driving the wheel in the reverse direction. The motors 18are driven by a hydraulic circuit which includes a pump 22 for supplyinghydraulic fluid under pressure selectively to fluid supply ports P1 andP2 respectively, the supply ports being connected to the motors 18 byhydraulic lines 24A, 24B, 24C, 26A, 26B and 26C.

As seen in FIG. 1, in this embodiment the inlet ports F, R of the rightfront wheel 12R are coupled to the supply ports P1; P2 of the pump 22via a selectively actuable switchover valve 30, whereas the inlet portsF, R of each of the other two wheels 12L, 14 are non-switchably coupledto the supply ports P1, P2 respectively. The operation of the valve 30is such that when the valve is not actuated (as shown in FIG. 1) theinlet ports F, R of the right front wheel 12R are connected to thesupply ports P1, P2 respectively, whereas when the valve is actuated(FIG. 2) the hydraulic lines 24B, 26B cross over so that the inlet portsF, R of the wheel 12R are connected to the supply ports P2, P1respectively. The valve 30 can be actuated and de-actuatedhydraulically, mechanically, electro-magnetically or in any othersuitable manner.

When the truck is configured for standard (i.e. forward/reverse)operation, FIG. 1, the valve 30 is not actuated and the rear wheel 14.isset substantially in the front-to-rear direction of the chassis 10. Inorder to drive the truck in the forward direction, fluid under pressureis supplied by the pump 22 to the supply port P1 and thus to thehydraulic lines 24A, 24B and 24C. Therefore, the fluid pressure isapplied to the inlet port F of each wheel motor 18, and all three wheelsare driven in a forward direction. In order to drive the truck in thereverse direction (not shown), the fluid under pressure supplied by thepump 22 is simply switched from the supply port P1 to the supply port P2and is thus removed from the inlet ports F and applied to the inletports R via the hydraulic lines 26A, 26B and 26C. The switchover of thehydraulic fluid under pressure between the supply ports P1 and P2 iseffected under operator control. In standard mode, in both forward andreverse drive directions, the front wheels 12L, 12R of the truck areturned to the front-to-rear direction and locked in position, the rearwheel 14 can be steered by the operator in conventional manner to steerthe vehicle, i.e. the rear wheel can turn clockwise or anti-clockwiseabout its axis 16 as the steering wheel (not shown) is rotated one wayor the other.

In order to configure the truck for operation in sideways mode, FIG. 2,the rear wheel is turned through 90° and locked, and the front wheelsare likewise turned inwards (i.e., in plan view the wheel 12L is rotatedclockwise about its axis 16 and the wheel 12R anti-clockwise about itsaxis 16), each through 90°, so that they lie in line and parallel withthe rear wheel (i.e. substantially normal to the front-to-rear directionof the chassis). Also, the valve 30 is actuated so that the hydrauliclines 24B, 26B cross over whereby the inlet ports F, R of the wheel 12Rare connected to the supply ports P2, P1 respectively. These actions maybe effected automatically by the vehicle control system (not shown) whenthe operator selects sideways mode, for example, by pressing a buttonlocated in the cab.

Now, if the hydraulic fluid under pressure is supplied to the port P1all three wheels, and hence the truck, will drive to the right, asindicated by the arrows in FIG. 2. Alternatively, if the hydraulic fluidunder pressure is supplied to the port P2, all three wheels will driveto the left (not shown). In sideways mode, in both left and right drivedirections, the front wheels 12L, 12R of the truck can be steered by theoperator, to change the directional course of the truck. Here, bothfront wheels rotate simultaneously in opposite directions, i.e. when onerotates clockwise about its axis 16 the other rotates anti-clockwiseabout its axis 16, and vice-versa.

The truck may also be configured for operation in carousel mode, FIGS. 3and 4. In carousel mode, the three wheels 12L, 12R and 14 are set atrespective directions in which their axes of rotation A1, A2, A3intersect at a substantially common vertical axis C equidistant fromeach wheel. Also, the valve 30 is not actuated. The vehicle controlsystem can place the truck automatically in carousel mode upon selectionof such mode by the operator (not shown), for example, by pressing abutton located in the cab.

Alternatively or additionally, the truck can be made to enter carouselmode from sideways mode by the operator turning the front wheels 12L,12R sufficiently that the two front wheels assume the relationship shownin FIGS. 3 and 4, as detected by for example, a sensor attached to oneof the front wheels. The sensor can be a proximity switch, a hydraulicswitch or any other suitable mechanism for determining when the wheelhas turned to a predetermined angle, and it may not be a wheel mountedsensor.

Then, if it is desired to rotate the truck in an anticlockwise directionabout the axis C, FIG. 3, hydraulic fluid under pressure is supplied tothe port P1. This fluid pressure is applied via the lines 24A, 24B and24C to the inlet port F of each wheel so that the wheels are driven inthe directions indicated by the arrows.

It will be seen that although the wheels 12R and 14 rotate in thecorrect direction to drive the truck anticlockwise about the axis C, thewheel 12L tends to drive the truck in the opposite direction. However,the combined torque of the wheels 12R and 14 substantially exceeds thatof the wheel 12L so that the wheel 12L is overcome and the truck as awhole rotates anticlockwise about the vertical axis C.

If it is desired to rotate the truck in a clockwise direction about theaxis C, FIG. 4, the hydraulic fluid under pressure is switched from theport P1 to the port P2. Now the fluid pressure is applied to the inletport R of each wheel so that all three wheels rotate in the oppositedirection to that shown in FIG. 3, i.e., the wheels are driven in thedirections of the arrows shown in FIG. 4. Again it will be seen thatcombined torque of two of the wheels 12R and 14 overcomes that of thesingle wheel 12L tending to drive the truck in an anticlockwisedirection so that the truck as a whole rotates in a clockwise directionabout the axis C.

In both anticlockwise and clockwise rotation all three wheels are drivenat all times, thereby overcoming the cavitation problem referred toearlier. Also, this embodiment takes advantage of the valve 30, which isalready present to provide sideways working, so that additional valvesare not necessary.

A second embodiment of the invention, FIGS. 5 to 8, is the same as thefirst except that a further selectively actuable switchover valve 40 isconnected between the inlet ports F, R of the left front wheel 12L andthe supply ports P1, P2 of the pump 22. In standard and sideways modes,FIGS. 5 and 6, the valve 40 is not actuated and the truck operates asdescribed for the first embodiment.

However, in carousel mode, FIGS. 7 and 8, the valve 40 is actuated sothat the left front wheel 12L, which in the first embodiment actedagainst the desired direction of rotation of the truck, now drives thetruck in the same direction as the other two wheels.

The invention is applicable to wheels driven by electric motors as wellas those driven by hydrostatic motors.

The invention is not limited to the embodiments described herein and maybe modified or varied without departing from the scope of the invention.

1. A four-directional forklift truck comprising a chassis having twofront wheels and one rear wheel, each wheel being directionallyadjustable by rotation about a substantially vertical axis, and arespective motor for driving each wheel selectively in either one of twoopposite directions of rotation, wherein the truck is operable in acarousel mode wherein the three wheels are set at respective directionsin which their axes of rotation intersect at a substantially commonvertical axis equidistant from each wheel and all three wheels aredriven, whereby the truck rotates substantially about the said commonvertical axis.
 2. A forklift truck as claimed in claim 1, wherein eachwheel motor is a hydrostatic motor, the truck further including ahydraulic circuit for supplying hydraulic fluid under pressure to eachmotor.
 3. A forklift truck as claimed in claim 2, wherein each motor hasfirst and second hydraulic fluid inlet ports, the application ofhydraulic fluid under pressure to the first inlet port driving the wheelin one direction and the application of hydraulic fluid under pressureto the second inlet port driving the wheel in the opposite direction,and wherein the hydraulic circuit comprises a source of hydraulic fluidunder pressure having first and second fluid supply ports, the hydraulicfluid under pressure being selectively supplied at the first or secondsupply port.
 4. A forklift truck as claimed in claim 3, in which thefirst and second inlet ports of at least one front wheel are coupled tothe first and second supply ports via a respective selectively actuableswitchover valve, and in which the first and second inlet ports of theother wheel(s) are non-switchably coupled to the first and second supplyports respectively, wherein when the truck is operated in standard modethe switchover valve is not actuated, when the truck is operated insideways mode the switchover valve is actuated, and when the truck isoperated in carousel mode the switchover valve is not actuated.
 5. Aforklift truck as claimed in claim 4, wherein the first and second inletports of only the said one front wheel are coupled to the first andsecond supply ports via a switchover valve whereby in the carousel modethe rear wheel and the said one front wheel drive the truck about thecommon vertical axis in a given direction of rotation against the actionof the other front wheel tending to drive the truck in the otherdirection of rotation.
 6. A forklift truck as claimed in claim 4,wherein the first and second inlet ports of the other front wheel arecoupled to the first and second supply ports via a respective furtherswitchover valve, the further switchover valve not being actuated in thestandard and sideways modes but being actuated in carousel mode, wherebyin the carousel mode all three wheels drive the truck about the commonvertical axis in a given direction of rotation.
 7. A forklift truck asclaimed in claim 4, wherein the truck enters the carousel mode fromsideways mode by de-actuating the switchover valve when the front wheelsare steered through a sufficient angle to set the three wheels atrespective directions in which their axes of rotation intersect at thesaid substantially common vertical axis.
 8. A four-directional forklifttruck comprising: a chassis having two front wheels and one rear wheel,each wheel being directionally adjustable by rotation about asubstantially vertical axis, a respective hydrostatic motor for drivingeach wheel selectively in either one of two opposite directions ofrotation, each motor having first and second hydraulic fluid inletports, the application of hydraulic fluid under pressure to the firstinlet port driving the wheel in one direction and the application ofhydraulic fluid under pressure to the second inlet port driving thewheel in the opposite direction, and a hydraulic circuit comprising asource of hydraulic fluid under pressure having first and second fluidsupply ports, the hydraulic fluid under pressure being selectivelysupplied at the first or second supply port, the first and second inletports of at least one front wheel being coupled to the first and secondsupply ports via a respective selectively actuable switchover valve andthe first and second inlet ports of the other wheel(s) beingnon-switchably coupled to the first and second supply portsrespectively, wherein when the truck is operated in standard mode theswitchover valve is not actuated, the front wheels are set substantiallyin the front-to-rear direction of the chassis, and the rear wheel issteerable to turn the truck in the required direction, and wherein whenthe truck is operated in sideways mode the switchover valve is actuated,the rear wheel is set substantially normal to the front-to-reardirection of the chassis, and the front wheels are steerablesimultaneously in opposite directions of rotation, the truck furtherbeing operable in a carousel mode in which the switchover valve is notactuated, the three wheels are set at respective directions in whichtheir axes of rotation intersect at a substantially common vertical axisequidistant from each wheel, and all three wheels are driven, wherebythe truck rotates substantially about the said common vertical axis. 9.A forklift truck as claimed in claim 5, wherein the truck enters thecarousel mode from sideways mode by de-actuating the switchover valvewhen the front wheels are steered through a sufficient angle to set thethree wheels at respective directions in which their axes of rotationintersect at the said substantially common vertical axis.
 10. A forklifttruck as claimed in claim 6, wherein the truck enters the carousel modefrom sideways mode by de-actuating the switchover valve when the frontwheels are steered through a sufficient angle to set the three wheels atrespective directions in which their axes of rotation intersect at thesaid substantially common vertical axis.